1. Field of the Invention
This invention pertains to conveyor belt idler assemblies. More specifically, the present invention pertains to conveyor belt idler assemblies of the type used to support trough-shaped bulk material conveyor belts, especially in loading zones.
2. General Background
Bulk material conveyor belts are typically supported by roller and/or slider bar idler assemblies arranged in a trough-shaped manner. In material loading zones, multiple idler assemblies are often arranged in series and in close proximity to each other to form impact cradles or beds that are capable of supporting the impact loads generated by material being loaded onto the belts. Ideally, the trough-shape formed by the idler assemblies should match the theoretical contour that the belt would naturally have given the load distribution on the belt (which, in general, is greatest at the center of the belt and follows a bell-shaped curve transversely across the belt). In actuality however, since the loading conditions on a given belt conveyor change during use, so too does the theoretical ideal profile of the belt. For this and other reasons, some idler assemblies (especially in high-load and/or high-speed applications) utilize what are often referred to as catenary idlers. A catenary idler assembly comprises a strand of several idler rollers and/or slider bars that are flexibly linked to each other. The strands are suspended such that the trough-shape formed by the idler assemblies can adjust in a catenary-like manner as the load distributions on the belt change.
Another driving force behind the design of impact-zone idler assemblies is the need to provide effective sealing. In belt loading zones, dust suppression and spillage are almost always a concern. To address this, conveyor belt systems typically are provided with generally fixed seals that slideably contact the upper surface of the belts near their outboard edges in loading zones. As can be appreciated, better sealing can be achieved by controlling the angular orientation of the outboard edge portions of the belt in the loading zone such that the angular orientations match the angular orientations of the sliding seals. Additionally, it is desirable to minimize belt sag between idler assemblies so that the edge margins of the belt are as linear as possible as the belt passes through the loading zone. These design goals conflict with the design goals of catenary idler assemblies since the goal of catenary idler assemblies is to allow the belt contour to adjust with changes in load distributions. Moreover, catenary idler assemblies generally are spaced farther apart from each other as compared to fixed idler assemblies to accommodate slight swinging of the suspended rollers. Spacing the idler assemblies farther apart can increase the degree of belt sag between the idler assemblies, which decreases belt sealing performance.
To address the conflict between the above-mentioned design goals, idler assemblies have been provided with fixed outboard or wing idler rollers or sliding bars, and with a catenary strand of idler rollers therebetween. In some cases, in addition to the normal flexibility of the catenary string, the catenary string is suspended from spring-biased mounting points to further improve the ability of the rollers to adapt to various contours and to reduce peak impact loads. However, utilizing such spring-biased catenary rollers in combination with fixed wing rollers or slider bars creates another problem. More specifically, as a spring-biased catenary strand of rollers changes the contour of the center of the belt, the outboard edges of the belt must move relative to each other. This means that there is transverse movement of the belt edge margins over the fixed wing idlers, which increases friction and belt wear.
Another problem associated with catenary idler assemblies is that catenary strands of idler rollers are inherently difficult to install or replace due to their flexibility and limp nature and due to the fact that the entire weight of a strand must be counteracted when installing the strand. These difficulties also increase the likelihood of injury to those providing installation or repair services.